1. Field of the Invention
The invention relates to a ball joint.
2. Description of Related Art
For example, at a junction between a knuckle arm of a suspension and a steering device, there is provided a ball joint (so-called outer ball joint) that supports the knuckle arm such that the knuckle arm is steerable and movable in the up-down direction. Some of such ball joints include a metal ball stud, a closed-end cylindrical housing, a resin seat made of synthetic resin, and a tubular rubber boot (dust boot) (See Japanese Patent Application Publication No. 2004-36700 (JP 2004-36700 A)). The ball stud includes a spherical head provided at a base end of its shaft. The housing holds the spherical head therein with the shaft projected from an opening of the housing. The resin seat is placed between the housing and the spherical head. The boot (dust boot) is attached to the housing. The ball stud is able to oscillate about the spherical head with respect to the housing, and is also able to rotate about the central axis of the shaft.
The boot covers the opening of the housing, and prevents dirt and water from entering the housing through the junction between the knuckle arm and the steering device. One end and the other end of the boot are fitted onto an open end of the housing (the end at which the opening is formed) and a middle portion of the shaft, respectively. Grease is provided between the resin seat and the spherical head, so the grease is sealed in the boot.
According to JP 2004-36700 A, an annular groove for fitting the boot to the housing is formed in the outer periphery of the open end of the housing, and one end of the boot is fitted in the annular groove. Further, an annular clip is wound around the one end of the boot. The one end of the boot is fixed to the housing by the clip. The annular groove is formed to have a generally rectangular shape in a section orthogonal to the radial direction thereof.
In the above-described ball joint, the ball stud oscillates with respect to the housing about the spherical head which acts as a fulcrum. The other end of the boot (the end portion of the boot, which is fitted onto the middle portion of the shaft) moves together with the shaft as the ball stud oscillates. Therefore, when the ball stud oscillates greatly, a portion of the boot, which is located on the side on which the shaft is tipped, is contracted, and another portion of the boot, which is located on the side opposite to the side on which the shaft is tipped, is stretched toward the other end side. Hence, as the ball stud oscillates, a load, which causes the one end (the end portion of the boot, which is fitted onto the open end of the housing) of the boot to move within the annular groove, is applied to the one end of the boot. If the one end of the boot, which is fitted in the annular groove, moves, the one end of the boot catches the grease sealed in the boot. This may cause leakage of the grease from the boot through the annular groove. In this case, the grease leaks from the boot because the one end of the boot moves easily within the annular groove. In order to reliably prevent leakage of the grease from the boot, it is desirable to increase a force for fixing the one end of the boot to the outer periphery of the housing.
In cold regions in particular, the viscosity of grease increases, and a significantly high load is required to tilt the ball stud and the boot. Therefore, when the ball stud oscillates, a load applied to the one end of the boot is significantly high, and the one end of the boot is displaced by a larger amount. Hence, in cold regions, the possibility of leakage of the grease from the boot may be high.
In order to prevent leakage of the grease from the boot, it is desirable that the one end of the boot and the end of the housing be in tight contact with each other and the other end of the boot and the middle portion of the shaft be in tight contact with each other. In order to achieve tighter contact between the one end of the boot and the end of the housing, for example, the surface roughness of the bottom face and a pair of side faces of the annular groove may be set relatively low (for example, the arithmetic mean roughness Ra may be set to a value lower than 12.5 μm).
The inventors of the invention have considered increasing the surface roughness of all the inner walls of the annular groove so that the frictional resistance between the one end of the boot and each of the inner walls of the annular groove is increased, thereby increasing a force for fixing the one end of the boot to the outer periphery of the housing. Thus, movement of the one end of the boot within the annular groove is suppressed.
However, if the surface roughness of the inner walls of the annular groove is high, small clearances may be formed between the inner walls and the one end of the boot. Therefore, increasing the surface roughness in all of the inner walls of the annular groove may cause leakage of the grease from the boot through the small clearances between the inner walls and the end of the boot.
Meanwhile, as described above, a portion of the boot, which is located on the side on which the shaft tips, is contracted due to oscillation of the ball stud. Due to the contraction of the boot, a middle portion of the boot moves toward one side in the axial direction of the housing, and a clearance is formed between the side face of the annular groove and the one end of the boot. As a result, the grease in the boot may enter the clearance and then leak from the boot.